Naltrexone is a current pharmacological treatment of narcotic addiction but its effectiveness is limited due to patient's unwillingness to accept and comply with dosage regimens that requires daily supervision in drug administration. Our long-term goal is to develop a sustained injectable drug for the treatment of narcotic drug addiction using the "Gel Depot" system, which can eliminate current limitations. Gel Depot is a novel, non-toxic, poly(ethylene glycol) based hydrogel system that is injected subcutaneously. This is in sharp contrast to other products under development based on irritating poly(lactic acid) and involves painful intra-muscular injections. We contend that use of naltrexone or methadone based Gel Depot can eliminate current limitation such as patient compliance since Gel Depot provides minimal burst effects, reduced potential for abuse and can be retrieved from patients if necessary. We propose in this exploratory proposal to test: i.) in vivo drug release in a rat model which will identify the optimal prototypes for maintaining therapeutic drug levels for at least 30 days; and ii.) Demonstrate the effectiveness of the gel in rats using the common behavioral procedures such as thermal nociception (Hargreaves device), mechanical stimuli and tail flick tests. We strongly believe that studies proposed here will lead to the development of simple but effective therapy in the treatment of opiate addiction, which still remains a major health and societal problem around the world.